The Surface Acoustic Wave (SAW) is an acoustic wave that follows a smooth boundary plane, with elliptical particle motion which is greatest at the surface and drops off so rapidly with a depth that almost all energy is carried in a one-wavelength layer at the surface. The SAW is used as the basis for a variety of electronic devices including delay lines, filters, and correlators which are key components in numerous electronic applications. Radio frequency (RF) SAW filters have a number of desirable characteristics with respect to their size and cost along with their electronic properties such as insertion loss, bandwidth, transfer function, and so on. Numerous advances in this field have led to greatly improved SAW device reliability and performance, and RF SAW filters have found widespread application in modern mobile communications equipment such as cellular telephones.
The Bulk Acoustic Wave (BAW) is a longitudinal or transverse wave that travels through solids essentially without boundaries. For example, the wavefronts extend over many wavelengths in all directions. The BAW has also been used in numerous electronic devices.
One problem impeding the continued development of SAW and BAW devices is the proliferation of unwanted spurious reflections that frequently occur in such acoustic devices. The deleterious effects from such unwanted reflections include the presence of delayed echo signals in the output of SAW delay lines and filters, the support of additional unwanted resonances in both BAW and SAW devices, and the conversion of leaked energy into additional unwanted modes in both BAW and SAW devices. The prior art has only enjoyed limited success in solving these difficult problems; and it is useful to illustrate the problem of unwanted spurious reflections.
Referring now to the drawings, FIG. 1 depicts a rectangular parallelepiped SAW substrate 1 with a single interdigitated transducer (IDT) 2 on the major surface of the substrate. The IDT 2 generates a SAW 3 that propagates along the major surface of substrate 1. When SAW 3 reaches the end 4 of the substrate 1, a reflected wave 5 reflects back to the IDT 2. The reflected wave 5 produces the unwanted spurious signal with the undesirable consequence of delayed echo signals in the outputs of SAW delay lines and filters. In addition, during reflection, a portion of the energy may be transferred into another wave of a different type that also perturbs the intended operation of the SAW device. A similar situation can arise with BAW devices where energy leaking laterally can reach the edges of the BAW device, leading to reflected waves that may become standing waves and conversion to other modes.
One prior art approach is to attempt to eliminate edge reflections by placing a lossy acoustic absorbing material on the substrate. FIG. 2 illustrates that approach. SAW substrate 10 is a rectangular parallelepiped with a single IDT 11 that generates a SAW 12, which propagates toward the end 13 of substrate 10. The energy of SAW 12 is absorbed by the lossy materials 14 placed on the end of substrate 10, and the reflected wave is eliminated. The main drawback with this approach is that the lossy materials 14 are usually incompatible with the required hermetically-sealed packaging of SAW and BAW devices where low aging is a requirement.
A second prior art approach is to alter the shape of the substrate as shown in FIG. 3. Referring now to FIG. 3, substrate 20 with IDT 21 generates a SAW 22 that propagates to end 23. In this case, the substrate 20 is not a rectangular parallelepiped, but rather has an end 23 that is cut at an oblique angle 24 with respect to the propagation direction of SAW 22. When the SAW 22 reaches end 23 with its oblique angle 24, the reflected wave, represented by broken line arrow 25, propagates at an oblique angle that induces a reduced spurious signal level as it passes over the IDT 21. A main disadvantage of this prior art approach is that the unwanted reflection 25 is only redirected, not eliminated, and it is therefore inapplicable to many potential integrated acoustic electronic applications wherein multiple SAW devices are located near each other on the same major surface of a common substrate. Furthermore, both prior art approaches suffer from the serious disadvantage of being incompatible with today's integrated microelectronics fabrication techniques.
Thus, there has been a long-felt need for a new approach to eliminating or substantially reducing the ill effects of unwanted spurious reflections in SAW and BAW devices that does not suffer from the undesirable drawbacks, limitations and shortcomings associated with lossy materials, redirected reflections, and incompatibility with integrated microelectronics technology.
Until now, there is no currently available, simple, low-cost and effective anti-reflection technique that enhances satisfactory acoustic performance and also avoids the disadvantages, shortcomings, and limitations of prior art devices.